QUESTION 83. _How is their steam-generating capacity increased above that of marine and stationary boilers?_ _Answer_. By creating a very strong draft of air through the fire and then passing the smoke and heated air through a great many small tubes, which are surrounded by water. By this means the smoke and hot air are divided into many small streams or currents which are exposed to the inside surface of the tubes to which and to the surrounding water their heat is imparted. QUESTION 84. _How is the action of the exhaust steam in producing a draft in the chimney explained?_ _Answer_. The exhaust steam escapes from the cylinders through one or two contracted openings or exhaust-nozzles (_f_, Plate II, also shown in fig. 40[21]), which point directly up the centre of the chimney or smoke-stack. The exhaust steam escapes from this orifice with great velocity, and expands as it rises, so that it fills the pipe _p_ and the smoke-stack _R R_. It thus acts somewhat like a plunger or piston forced violently up the chimney, and pushes up the air above it, and, owing to the friction of the particles of air, carries that which surrounds it along up the stack, from which it all escapes finally into the open air, thus leaving a partial vacuum behind in the smoke-box. The external pressure of the atmosphere then forces in air through any and every opening in the smoke-box, to take the place of that already drawn out or exhausted from it. As the only inlet is through the tubes, to which the gases of combustion have free access from the fire-box, and as the external air can only pass through the fire-grate, and through the burning fuel, to reach the fire-box, there is a constant draft of air through the grate as long as the waste steam escapes from the blast-pipe and up the chimney. It is thus that, within certain limits, the more the steam that is required, the more the steam that is produced; for all the steam used in the engine draws in the air in its final escape, to excite the fire to generate more steam.[22] Sometimes one blast-orifice is used for each cylinder, as shown in plates II and III and fig. 40; in other cases the exhaust steam from each cylinder escapes through the same orifice. [21] The term _blast-orifice_ is also often used to designate these parts of locomotives. [22] Colburn’s Locomotive Engineering. QUESTION 85. _How much water is it necessary to evaporate in order to furnish the steam required to run an ordinary train at its usual speed?_ _Answer._ For an ordinary “American” locomotive,[23] weighing 60,000 lbs. and with cylinders of 16 inches diameter and 24 inches stroke, from 6,000 to 12,000 lbs. of water must be evaporated per hour. [23] In speaking of “American” locomotives, we mean locomotives like that shown in Plate I, with four driving-wheels and a four-wheeled truck, and shall so use the term hereafter. QUESTION 86. _How much water will a pound of coal evaporate in ordinary practice?_